Which vitamin is essential for gamma-carboxylation of clotting factors II, VII, IX, and X?

• A. Vitamin A
• B. Vitamin D
• C. Vitamin K
• D. Vitamin C

Answer: (C)

Gamma-carboxylation of certain clotting factors relies on vitamin K to activate the enzyme that converts specific glutamate residues to gamma-carboxyglutamate. This modification creates calcium-binding sites on factors II (prothrombin), VII, IX, and X, which is essential for their proper assembly on phospholipid surfaces and for efficient coagulation. Vitamin K cycles between reduced and epoxide forms to sustain this process; disruption of this cycle (as with warfarin) impairs gamma-carboxylation and anticoagulates. Other vitamins like A, D, and C do not participate in this carboxylation step, so they don’t enable the activation of these clotting factors.